by signing up with Facebook: sign up with facebook on the new app. The system will pop you the old tactical map username, which you will be able to confirm.

by signing up with your email: sign up with your email, redo the registering process with the email you are trying to log in with and confirm the proposed username. You should be able to get an email with a verification link to click on, so after that you should restart the app with the newly received details.